Research On Rapid Methods For Measuring Radioisotope Strontium Activities In Different Matrices

Among the productions of nuclear fission,the radioactive isotopes of strontium are hazardous,since the two common isotopes,Sr-89 and Sr-90,have relatively high fission fractions and they are easily accumulated by human bodies.Therefore,it is of great significance to accurately and quickly determine the activities of Sr-89 and Sr-90 in both daily and emergency monitoring.At present,the main analysis method for radioactivity of Sr-90 in the environment in China is based on the "Environmental radiation monitoring analysis method and management standard compilation"published in 1989(abbreviated as the GB method).The principle of the GB method is to separate and purify Y-90 which has reached the equilibrium state through chemical analysis,and then measure the Y-90 activity using a low background ?/? detector to determine the activity of its parent Sr-90.The GB method takes a long time since the secular equilibrium between Sr-90 and Y-90 requires sufficient time.In addition,it can only analyze the activity of Sr-90.Therefore,the GB method does not meet the requirements in emergency monitoring of nuclear incidents that have the need to rapidly and simultaneously measure a variety of radioactive isotopes of strontium(such as Sr-89 and Sr-90).In this dissertation,based on the study of the characteristics of radioactive strontium,we systematically designed new methods for rapidly measuring radioisotope strontium activities in different matrices,such as water,soil,biological samples and sediments,after summarizing the advantages and disadvantages of the existing methods in the world.The rapid method proposed in this dissertation uses the crown ether(Sr resin)to separate and purify the Sr2+ ions.Afterwards,the liquid scintillation detector is used to measure radioisotope strontium activities in the purified sample.Finally,the spectral analysis method is used to calculate and determine the activities of Sr-89 and Sr-90 simultaneously.In order to determine the best experimental conditions and detection performance,studies on experimental materials and equipment were first performed.We have analyzed the adsorption and desorption properties of Sr resin with respect to acidity,temperature,oscillating time,flow rate,release agent,desorption flow rate,the amount of eluent and other factors,and explored the effects of different types of liquid scintillation container,scintillation fluid volume,sample volume,PSA value,efficiency scale and quenching calibration curve and other parameters when using the liquid scintillation detector.Then,the three spectral analysis methods,least squares inverse matrix method,stripping spectrum method and spectral fitting method were described in the dissertation.By employing these methods,we can directly determine the activity of each radionuclide in the mixed spectrum.Finally,for the four matrices commonly used in environmental samples analysis(liquid,biological ash,soil and sediment)systematical rapid methods for measuring radioisotope strontium activities have been proposed.Moreover,this dissertation also describes a related method for using liquid scintillation detectors in the Cerenkov counting mode,which is a supplement to the rapid methods.The performance of the experimental materials shows that Sr resin has good adsorption capacity for Sr2+ in acid atmosphere and within a certain range,the stronger the acidity,the stronger the resin's ability to adsorb Sr2+.In weak acids or deionized water,Sr2+ can be effectively desorbed from the resin.The entire separation process is not affected by the flowing rate,so the separation time is short.In order to verify the proposed rapid methods,we conducted numerous experiments.We have selected multiple sets of environmental samples and standard samples with known activities.Each sample was repeatedly measured by the rapid method,the GB method,and the Cherenkov method.Afterwards,the relevant experimental results were compared.According to the comparisons,the experimental results obtained by different methods show a high degree of consistency with an average relative deviation smaller than 10%.These results validate the accuracy and effectiveness of the rapid method proposed in this dissertation.Moreover,by comparing the experimental results,we found and verified that the GB method could not eliminate Bismuth in the samples.We have described an improved GB method.A joint measurement of Sr-90 and Pb-210 in environmental samples was briefly introduced,and the experimental results were in good agreement.We also calculated the minimum detectable activity(MDA)of the rapid measurement method,which is 2.1mBq/L(Kg).The rapid method proposed in this dissertation has the following advantages:1)The entire analysis process only takes 6 hours,which is 60 times faster than the GB method;2)Microwave digestion is used to digest the solid sample,reducing the required amount of the sample and making the entire pretreatment process faster,cleaner,and thorough.It also avoids the harm of the acid mist to the operator's body during the pretreatment process using the acid in the GB method;3)the liquid scintillation detector is used for measuring the radioactivities.With the spectral properties of the liquid scintillation detector,one can directly observe whether the radionuclide to be analyzed is completely purified.At the same time,spectral analysis method can be used to analyze the activities of multiple radionuclides simultaneously so as to avoid repeated chemical pretreatments.In summary,a rapid analysis method for measuring radioisotope strontium activities in environmental samples in different matrices has been described in this dissertation.This new method significantly reduces the time required,simplifies the chemical treatment procedures,and can simultaneously detect multiple different radionuclides.It fully meets the requirements in daily environmental monitoring and emergency monitoring of nuclear incidents.